Although there are a myriad of different types of stuff sacks ranging from the high-end ultra-light ones (e.g. this one, or this one) that weigh less than an ounce, to the regular, run of the mill ones (e.g. this one) that weigh a couple of ounces. The problem with these is that, the high-end ones are expensive and the cheap ones are "heavy." So, what's the solution that trades time for money and sprinkles in a dash of fun? DIY, of course. The instructions below are for making silicon-nylon (silnylon) stuff sacks that are quite light and cheap. The 12L bag from the instructions below (8" wide by 16" deep) only weighs 5/8th of an ounce and cost me about $3 in materials (I bought about $12 worth of material, and with that I should be able to make at least 4 sacks.)
The instructions that you will find below are originally from the Thru-Hiker site found here, but I've taken a couple of more pictures that might make the instructions a bit clearer.
Cut your fabric to size
For a roughly 12L bag measuring 8 inches wide by 16 inches deep, cut a 21 inch by 26 inch piece of fabric. All steps will be using a 1/4 inch seam allowance, so adjust this number if you want a larger/smaller seam allowance.
Sew 2 sides
Fold the fabric in half along the longer 26 inch length side so that what will be inside when you are done is on the outside. In other words, if you are using some coated material, make sure the coated side is out (so that it is on the inside when you are done). Sew the side and bottom with a simple seam using a 1/4 inch seam allowance.
Making the Bottom
Turn the sack so that you are looking at the bottom, and pull the two sides apart. You should be looking at the seam you just sewed. Using your ruler, measure the length of the bottom seam and divide the length by 4. Now, mark the points on the seam that are 1/4 of the away from each edge of the seam. For the measurements above, the bottom seam should be just about 12 inches, so mark a point 3 inches from either end on the seam. Using your ruler, make a line (or mark the points) perpendicular to the seam at the two points your just marked.
Making the Drawcord Channel
Sew a hem
Remove the accessory tray from your sewing machine and run a 1/4 inch hem along the top of the stuff sack. Although not necessary, for the sake of neatness, I start and end the stitching of the hem at the seam that I sewed at the very beginning. Once you're done, replace the accessory tray.
Sew a button hole
Now, for the hardest part. We need a way for the drawcord to come of out the drawcord channel, and for this we will sew a 1 inch button hole (you can make it slightly smaller, but anything less than 3/4 inch and you won't be able to pull the cord through). I also use a piece of extra material (from the pieces cut off from the bottom) to reinforce the hole. If you prefer, you can also use a metal grommet, but sewing the button hole is both lighter and less likely to break in the future. The stuff sack looks the nicest if you place this button hole opposite from the seam down the side of the sack, but functionally it doesn't matter where you place it (except for on the seam). If your sewing machine has a button hole setting, change your presser foot and select the button hole setting. If not, you can follow the instructions here that show you how to make a manual button hole. A note of caution for those of you using the button hole setting and button hole presser foot. The silnylon material tends to be very slippery, so you may find that the presser foot doesn't move along properly with each stitch, possibly resulting in a larger than expected button hole. If that is the case, you can manually lift the button hole presser foot and adjust its location so that you get the appropriate sized button hole.
Sew the drawcord channel
Remove the accessory tray from your sewing machine once again, and sew the drawcord channel by folding over the top of the stuff sack. If you made a 1 inch button hole, the channel should be about 1/2 inch wide. Again, for the sake of aesthetics, I start and end the sewing right about where the button hole is. Also, since there already is a line of stitching from the original hem, try to sew on or as close to that hem as possible.
Inserting the drawcord
You can now turn your bag inside out, so that the right side is out for the first time. Quickly admire what you have done. Now, take the drawcord and pull it through the channel. I found that the easiest way to do this is the attach a safety pin (the larger the better) to the cord and feed it through the channel by bunching up the fabric in front of the safety pin and then pulling it over the safety pin. You can either cut the drawcord to length before or after you pull it through.
Add the cord lock. Depending on how small your cord lock is, it may be hard to pull the two ends of the cord through the lock. I found that by fusing the two cord ends together using a lighter makes it much easier to push through the cord lock. If you didn't fuse the drawcord to pull it through the lock, make sure to melt the ends to make sure that they don't fray. That's it. You can now admire your new stuff sack!
(Optional) Seam sealing
If you want to have a water-proof stuff sack, it's now time to seam seal all of the seams. So, follow the instructions on the seam sealer that you have and seal the seams. Voila, you're done.